1. Field of the Invention
The present invention relates to a transition metal-substituted hydrotalcite catalyst for removing nitrogen oxides from the exhaust gas of diesel engines by NOx storage and reduction (NSR).
2. Description of the Related Art
In diesel engines, which are operated in the presence of excess oxygen to have high combustion efficiency, unlike gasoline engines, carbon monoxide, unburned hydrocarbons, and nitrogen oxides cannot all be removed using three-way catalysts because the concentration of oxygen included in the exhaust gas of diesel engines is high. For this reason, a selective catalytic reduction (SCR) method of removing nitrogen oxides by supplying urea as a reductant and an NOx storage and reduction (NSR) method of removing nitrogen oxides by reducing nitrogen oxides in an oxidation atmosphere after storing nitrogen oxides and then injecting fuel have been introduced. The NSR method can be easily applied to diesel automobiles because fuel is directly sprayed, and thus the addition of reactants is not required, with the result that additional facilities or apparatuses are not required.
Generally, the NSR catalyst is formed of precious metals, performing oxidation-reduction, and materials for storing nitrogen oxides. In an oxidation atmosphere, nitrogen oxides are oxidized to nitrogen dioxide by precious metal components, and then the nitrogen dioxide is stored in barium oxides, and, in a reduction atmosphere, in which fuel is sprayed, the stored nitrogen dioxide is desorbed, and the desorbed nitrogen dioxide is reduced to nitrogen in the presence of a precious metal catalyst. Accordingly, in order for the NSR catalyst to have excellent performance, in an oxidation atmosphere, a large amount of nitrogen dioxide must be stored, and, in a reduction atmosphere, nitrogen dioxide must be rapidly reduced. Further, the NSR catalyst must have excellent stability in order to be used for a long time in consideration of the properties of catalysts for purifying the exhaust gas of automobiles, which cannot be easily replaced. Since the exhaust gases of diesel engines are combustion products, and thus include a large amount of water, the NSR catalyst cannot help but be exposed to water for a long time. Since the NSR catalyst cannot be easily replaced, the NSR catalyst must have excellent hydrothermal stability in order to be stably used for a long time. Further, when sulfur compounds included in diesel fuel oil are combusted, sulfur dioxide is produced. Since sulfur dioxide has chemical properties similar to those of nitrogen dioxide, it is adsorbed to the nitrogen oxide adsorption sites of the NSR catalyst. Generally, since sulfur dioxide has stronger adsorbability than nitrogen dioxide, the adsorbed amount of nitrogen dioxide cannot help decreasing due to the adsorption of sulfur dioxide.
Accordingly, the preferred NSR catalyst for diesel engines must store a large amount of nitrogen dioxide in an oxidation atmosphere, and must easily reduce desorbed nitrogen dioxide in a reduction atmosphere due to the high dispersity and stability of precious metals. Further, in order for the NSR catalyst to be used as a commonly-used catalyst, the NSR catalyst must have excellent hydrothermal stability so that it can be used without being replaced for a long time, and must have excellent properties of resistance to sulfur dioxide poisoning.
The NSR catalyst is made of strongly basic materials in order to store nitrogen dioxide, which is acidic. Further, the NSR catalyst is made of water-stable materials so that it is not deformed or melted by aqueous vapor included in exhaust gas. For this reason, initially reported NSR catalysts were manufactured by supporting water-insoluble and strongly basic barium oxides in thermally-stable alumina having a large surface area. In this case, in order to increase basicity, potassium oxides can be added thereto. However, since alkali metal oxides are easily dissolved in water, the increase in basicity due to the addition of alkali metal oxides has been limited.
Meanwhile, hydrotalcite is carbonate of magnesium and aluminum, having hydroxide groups, and is generally represented by the molecular formula: [M1−x+2Mx+3(OH)2]x+[Ax/nn−]·mH2O. Here, M+2 and M+3 are divalent and trivalent cations, respectively, and typical elements thereof are magnesium and aluminum, respectively. A is an interlayer anion, and may be a carbonate ion. A brucite structure, including a cation formed by partially substituting a brucite structure of magnesium hydroxide with an aluminum ion, is bonded with an anion and water.
Divalent or trivalent metal ions having an ion radius similar to magnesium or aluminum can form a hydrotalcite structure. Metals, such as calcium (0.99 Å) or beryllium (0.35 Å), having an ion radius much greater or smaller than magnesium, cannot form a hydrotalcite structure. In contrast, metals, such as copper, nickel, cobalt, zinc, iron, gallium, chromium, manganese, and the like, having an ion radius similar to magnesium, replace magnesium or aluminum, thus forming a hydrotalcite structure. The composition ratio of divalent ions and trivalent ions is also important to the formation of a hydrotalcite structure. When the molar ratio [M3+]/([M3+]+[M2+]) thereof is in the range of 0.20 to 0.33, a pure hydrotalcite structure is formed ([1] A. Vaccari, “Clays and catalysis: a promising future”, Appl. Clay Sci., 14, 161(1999)).
The size of anion is not particularly limited either, but a hydrotalcite structure can be formed of inorganic anions (fluorine, chlorine, bromine, nitric acid, and carbonic acid) and organic acids (adipic acid, oxalic acid, and malonic acid). The number and bonding force of anions bonded with brucite cations change depending on the kind of anion. The surface area of the synthesized hydrotalcite is in the range of 100 to 300 m2/g, which is very large. The hydrotalcite functions to exchange anions because anions are included therein. The hydrotalcite can also be used for base catalysts because it has strong basicity. Further, the hydrotalcite is an oxide having a uniform composition and a predetermined structure because it is a crystalline material ([2] A. Vaccari, “Preparation and catalytic properties of cationic and anionic clays”, Catal. Today, 41, 53(1998)).
The hydrotalcite can store nitrogen dioxide because it is a basic material. When the backbone thereof is substituted with metals having atomic radii similar thereto, the basicity thereof is changed. The hydrotalcite is stable in hydrothermal treatment and can widely disperse and support precious metal because it is bonded with hydroxide groups and has uniformly dispersed constituent atoms ([3] E. Kanezaki, “A thermally induced metastable solid phase of Mg/Al-layered double hydroxides by means of in situ high temperature powder X-ray diffraction” J. Mater. Sci. Lett., 17, 371 (1998)).